In structure panel, mark items not accessible outside the module with a "private" icon

[jfehrle]

An .ml file can have functions or variables that aren't accessible outside that file because they're not listed in the corresponding .mli file. When I browse .ml files looking for routines that I might be able to call from outside the file, I'd like to immediately see which things are available and which ones are not (aka private). This could be shown in the structure panel with the lock icon and maybe also in the left margin on the definition of the item (In Java it's obvious from the private but not obvious in OCaml.)

Similarly, this should apply to submodules within a file where the struct has definitions not included in the module signature in this general form:

module Foo : sig
  <equivalent to .mli here>
end = struct
  <equivalent to .ml here>
end